(1) Field of the Invention
The invention relates to railguns and, more particularly, railgun armatures having a conductive lubricant and system of delivery which reduces the electrical resistance and friction of the armature-rail contact, thereby reducing the amount of heat generated at the armature-rail sliding electrical contact.
(2) Description of the Prior Art
A railgun is an entirely electrical gun that accelerates a conductive armature along a pair of rails to launch a projectile. A typical railgun includes at least one pair of oppositely spaced, generally parallel electrically conducting rails. The breech ends of the rails are connected to a pulsed-power current source, typically a bank of capacitors or a large homopolar generator. A projectile is placed between the rails. When the current pulse is applied to the rails, the conductive armature completes the current path between the rails and accelerates the projectile. Particular characteristics of a railgun are the lack of propellant (only the projectile, armature and the electrical energy to launch it are required to be expended) and the ability to launch projectiles much faster than chemical propellant-based technology allows.
Railguns are being researched as weapons with projectiles that do not contain explosives, but are given extremely high velocities—about 3,500 m/s or more, making the railgun's kinetic energy equal or superior to the energy yield of an explosive-filled shell of greater mass. This would allow more ammunition to be carried and eliminate the hazards of carrying explosives in a ground vehicle or naval vessel. Also, by firing at greater velocities, railguns have greater range, less bullet drop and less wind drift, bypassing the inherent cost and physical limitations of conventional guns.
Intense heating of the armature-rail sliding contact due to electrical resistance and friction damages the rails in electromagnetic railguns and limits their service life.
Solid railgun armatures are fabricated from aluminum alloys due to their low density and melting point. Melting of the aluminum armature at the armature-rail contact is thought to produce a thin liquid metal layer, which is observed on the rails as a transfer film. The hot liquid aluminum is metallurgically reactive and forms brittle intermetallic compounds at the rail surface, which may lead to runaway heating and transition to a plasma state at the contact. The high temperature plasma is damaging to rails and insulators in a railgun. Moreover, the liquid aluminum forms small droplets which burn violently in contact with air. The burning droplets form aerosolized aluminum oxide particles which may pose a respiratory hazard.
Georgia Tech undertook a study of liquid gallium as a railgun lubricant at the same time Northwestern University, working with the University of Texas at Austin, investigated bismuth, tin and indium as low melting point solid lubricants. Liquid gallium and mercury are known to embrittle high performance alloys at room temperature, and require protective coatings to prevent catastrophic failure of the armature during launch (due to the loss of ductility). Moreover, gallium is mildly corrosive and requires personal protective equipment in handling. The bismuth, tin and indium elements and binary alloys of these elements studied at Northwestern University are nontoxic and do not embrittle aluminum in their solid form. However, it is necessary to melt the elements and their binary alloys to permit casting into armature reservoirs. This is a difficult operation when one side of the armature is already filled, since the armature temperature must not exceed the melting point of the element or alloy. Channels and large rectangular surface reservoirs were tested on a small-caliber electromagnetic launcher at the University of Texas at Austin's Institute for Advanced Technology.
More recently, the Naval Research Laboratory tested mercury as a liquid railgun lubricant. Due to the acute and chronic toxicity of mercury and its chemical compounds, it is unsuitable for use where personnel may be exposed to lubricant residues.
There is a need for a conductive lubricant and system of delivery which reduces the electrical resistance and friction of the armature-rail contact.